Physiology, Osmoregulation and Excretion

As living organisms, the maintenance of fluid balance is critical to sustaining many bodily functions, including metabolic and biochemical reactions, transport of nutrients and thermoregulation. The average adult has roughly 65% fluid mass, with this value being slightly lower in females than males. Our body fluids may subcategorize into intracellular and extracellular fluid compartments. [1] [2] [3] Intracellular fluid, also considered as the cytosol, is all the fluid contained within cells. The intracellular fluid makes up roughly two-thirds of the total body volume.Extracellular fluid constitutes the remaining one-third of fluid volume in the body and may further divide into its interstitial, intravascular and transcellular compartments. [1] [2] [3] The maintenance of fluid homeostasis in each of these compartments is dependent on the excretion of fluids and the concentration of electrolytes that generate osmotic pressure. This process of passive regulation of osmotic pressure is known as osmoregulation. [1] [2] [3] This article will provide an overview of osmoregulation and excretion, focusing on a discussion of the renal system involved in maintaining this intimate balance between fluid retention and excretion.